In many applications, it is preferred if all of the different soil and crop requirements for individual nutrient components (based on potassium, nitrogen and phosphorus) are in the form of granules which are approximately similar to each other in terms of dimensions and physical characteristics so that a composite fertilizer, which is generally homogeneous, may be obtained when such granules are mixed. Additionally, it is preferred if such fertilizers may be manufactured in such a way that the release of the nutrient from the fertilizer granulars may be controlled. One method of making a fertilizer controlled release is to coat it with a material that gives controlled release characteristics. This can be achieved in many ways including coating with a polymer in a fluidized or spouted bed.
In many of the traditional fertilizer preparations, the potassium nutrients are provided in the form of potassium chloride. In certain applications, low chloride or no chloride fertilizers are desirable. For example, in the nursery business, some species of potted plants are chloride or salt sensitive and thereby require a fertilizer having little or no chloride. Additionally, in high salt areas such as found near the ocean, it is desirable to use fertilizers which do not contribute further to the salt problems. In many such cases where a soil is high in salt, plant growth can be impeded due to the dehydration if a chloride fertilizer is used.
One such chloride-free compound for use in providing potassium nutrients in fertilizer applications is potassium sulfate. However, it was found that all commercially available potassium sulfate was totally unsuitable for applications where it would be subject to crushing and abrasion such as in the process of coating for controlled release applications or in rigorous mixing or blending applications. In particular, it was found that such commercially available potassium sulfate deteriorated into dust or broke into pieces during coating or other processing that subjected the granules to impact and abrasion.
To be suitable for coating, granules have to be round, in an acceptable size range suitable for blending, and have a hardness and abrasion resistance which prevents the granules from breaking up under the rigorous handling of the coating process or under rigorous mixing or blending.